This invention relates to rolling mills, and more specifically to a multiple-staged rolling mill effective for the shape control of the material to be rolled.
It is well-known with rolling mills for strips that the upper and lower work rolls are bent and deflected by the reaction force acted upon by the strip, with the result that the strip tends to develop defects of shape, such as edge wave or elongation, plate crown in which the longitudinal middle portion of the strip is increased in thickness, and so on, upon rolling. Also it is known that during high speed rolling or in rolling light metal materials the contacting surfaces of the work rolls are so deformed by thermal expansion as to make the strip ununiform in thickness across its width, thinning the middle portion of the strip and thereby causing middle elongation and other defects.
Attempts to prevent those defects of shape have been made. For example, rolling mills of recent designs include roll benders connected either to the upper and lower work rolls or to the upper and lower backup rolls so that increased or decreased bending forces may be applied, when necessary, on the work rolls or backup rolls. Those roll benders, which cause changes in the work roll crown according to the cross sectional profile of the strip or the rolling force used for the rolling operation, are generally believed fairly helpful in avoiding the defects of shape in the strip upon rolling.
However, the experience in the past years with those mills incorporating roll benders has revealed that the increased bending forces applied by the benders associated with the work rolls or backup rolls are not in the least effective in arresting complex elongation, or the combination of edge and middle elongations, even though the forces may suppress the edge waves along. It has also been empirically found that decreased bending forces produced by the benders connected to the work rolls are substantially ineffective in controlling the middle and edge elongations.
The problems of the existing mills equipped with roll benders will now be more fully analyzed. First, discussion will be made in connection with a four high mill of a conventional design having roll benders connected to the upper and lower work rolls so as to apply increased bending forces on those rolls.
As is commonly known, the upper and lower work rolls during rolling operation are deformed by the reaction force of the strip being rolled, with the middle portions of those work rolls bent away from each other. As a result, the strip tends to be rolled to an increased thickness in the middle portion across its width, producing waves along the both edges. In an effort to alleviate these unfavorable tendencies, modern mills have roll benders connected to the both ends of the work rolls and thereby apply increased bending forces in such a way as to force the both ends of the upper and lower work rolls away from the strip to be rolled. The application of increased bending forces on the work rolls is expected to correct the axis of the upper work roll, allowing it to draw either a rectilinear line or a secondary line convexed downwardly. Actually, however, the axis draws a wide-W-shaped curve, convex upwardly in the middle of the roll and concave in the both end portions. Accordingly, the edge portions of the strip, each extending over about one-fourth of the total width, are subjected to intense rolling forces and therefore tend to develop complex waveness or elongations, or a combination of edge and middle waves, upon rolling.
In order to deflect the upper work roll until its axis draws a downwardly convex secondary curve, it is necessary to deflect the upper backup roll, too, because the work roll is under constraints of its backup roll and the strip being rolled. However, it must be noted that the backup roll has by far the greater rigidity than the work roll and is subjected at both ends to a heavy load which is the sum of the rolling load and the increased bending forces applied. Thus, the axis of the upper backup roll is deflected, drawing an upwardly convex secondary curve. Now if increased bending forces are given to the upper work roll, that roll and the upper backup roll will be pressed hard against each other in the portions where they are free from the reaction force of the strip. Those portions of the upper work roll are deformed by the upper backup roll acting as levers, while the middle portion of the upper work roll constrained by the upper backup roll and the strip does not draw a downwardly convex secondary curve. After all, the upper work roll is deformed so that its axis forms a wide-W-shaped curve.
With a prior art four high mill of the modified design having roll benders connected to both upper and lower backup rolls in order to apply increased bending forces on those rolls, the increased bending forces cause the backup rolls to deflect at the opposite ends away from the strip, thus solving the afore-described problems of the conventional four high mill having work rolls equipped with roll benders. Still, the modified mill in turn poses new problems as follows:
Next, a four high mill of the type including benders connected to the upper and lower work rolls so as to apply decreased bending forces on those rolls will be considered.
In case of high speed rolling or in rolling light metal materials on this type of mill, the heat generated by rolling causes the work rolls to expand particularly in the middle portion. The strip accordingly tends to be rolled with decreased thickness in the middle portion, resulting in middle elongations or waviness. In order to repress this tendency, modern mills include roll benders connected to the both ends of the upper and lower work rolls and adapted to apply decreased bending forces to the work rolls so as to force the both ends of the rolls toward the strip to be rolled. It is intended by the application of decreased bending forces to make the surface of the upper work roll in contact with the strip substantially rectilinear or convex upwardly. Actually, on the contrary, the roll is deformed to a generally wide-W-shaped curve, convex downwardly in the middle and upwardly in the both end portions. Middle and edge waves, therefore, are left behind in the rolled product. The reasons for which the contacting surface of the upper work roll is deformed in such a manner have already been explained and the explanation is not repeated here.
As regards this problem, the same may be said of a rolling mill of the type having benders connected to the upper and lower backup rolls to apply decreased bending forces to those rolls.